Proximity switch having a magnetic shield



J- P. KO HLS PROXIMITY SWITCH HAVING A MAGNETIC SHIELD Oct, $5, 1968 2Sheets-Sheet 1 Filed Aug. 215, 1966 1 |||l| /II//// I, I 1 ill llh I 1 11 F l G. 2

mvewron 4,415: Ao/as BY AZ 41.; 4 41 A TTORNEVS Oct. 15, 1968 J. P.KOHLS PROXIMITY SWITCH HAVING A MAGNETIC SHIELD 2 Sheets-Sheet 2 FiledAug. 215, 1966 FIG.3

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INVENTOR JAMES P AOl/ZJ A T TORNEVS United States Patent 3,406,267PROXIMITY SWITCH HAVING A MAGNETIC SHIELD James P. Kohls, Detroit,Mich., assignor to Jervis B. Webb Company, a corporation of MichiganFiled Aug. 23, 1966, Ser. No. 574,395 Claims. (Cl. ZOO-61.41)

This invention relates to an improved construction for a proximityswitch of the type employing a switch element having magneticallyoperable contacts mounted in a magnetic field formed by biasing andsensing magnets so that the relative position of the switch contactswill be changed when a ferromagnetic object is brought within themagnetic field.

The operating characteristics and sensitivity of a switch of this typeare of course determined by the manner in which the switch is designedand manufactured, and these characteristics are set when the switchleaves the factory. When the switch is used, it must be mounted on somestructure and if this structure is of iron or steel, then it is usuallynecessary to employ a mounting member of non-magnetic material in orderto space the switch a minimum distance from the structure of magneticmaterial in order that the sensitivity and operating characteristics ofthe switch will not be affected thereby. Even after the switch has beeninstalled, its operating characteristics can be affected by any changein the amount or positioning of surrounding magnetic material. Theseconsiderations have tended to reduce the reliability and usefulness ofmagnetic proximity switches in many possible industrial applications.

One object of the present invention is to provide a magnetic proximityswitch of the type under discussion which can be mounted directly onstructure of ferromagnetic material, and which is insensitive to thepresence of ferromagnetic material beyond a location defined by a shieldplate incorporated in the magnetic circuit of the switch.

Another object is to provide an improved arrangement for biasing theswitch for a normally open or normally closed condition of the switchcontact elements and for adjusting the sensitivity of the magneticcircuit of the switch to the presence of magnetic material to bedetected, with the aforementioned shield plate forming part of theimproved biasing arrangement.

A magnetic proximity switch constructed in accordance with the inventionincludes a non-magnetic case having a sensing face and a switch elementwith a pair of magnetically operable contacts mounted in a sensingmagnetic field and in a biasing magnetic field. The sensing magneticfield is formed by a pair of sensing magnets mounted in the case inspaced relation to each other with one pair of opposite poles adjacentthe sensing face, the magnetically operable contacts of the switchelement being mounted in the field between the other pair of oppositepoles. The biasing magnetic field is formed by a pair of biasing magnetspositioned in the case with one pair of opposite poles adjacent themagnetically operable contacts of the switch element. A shield member ofmagnetic material is mounted in the case to form a closed flux path forthe sensing and biasing magnetic fields, which closed flux path isspaced from the sensing face and shields both the sensing and biasingmagnetic fields from the influence of extraneous magnetic material otherthan magnetic material to be detected adjacent the sensing face of thecase.

In one preferred construction, the shield member is provided with asurface on which the biasing magnets are mounted for movement toward andaway from each other to vary the biasing magnetic field relative to themagnetically operable switch contacts whereby the 3,406,267 PatentedOct. 15, 1968 normal position of the switch contacts can be varied andthe sensitivity of the proximity switch to magnetic material adjacentthe sensing face of the case can be adjusted.

Other features and advantages of the invention will appear from thefollowing description of the representative embodiments disclosed in theaccompanying drawings in which:

FIGURE 1 is a plan view of a switch with the cover of the case removed;

FIGURE 2 is a sectional elevation taken as indicated by the line 22 ofFIG. 1 but showing the cover of the switch in place and the switchmounted on a structural member;

FIGURE 3 is a plan view similar to FIG. 1 showing an alternateconstruction; and

FIGURE 4 is a sectional elevation similar to FIG. 2 but taken on theline 4-4 of FIG. 3.

The switch construction disclosed in FIGS. 1 and 2 includes a case 10 ofcast aluminum or other suitable non-magnetic material having a centralbafiie 11 which divides the interior of the case into two compartments.One compartment 12 includes bosses 13 and 14 for mounting bolts 15 whichalso retain a cover plate 16, and receives the structural and electricalleads 17 and 18 respectively for the switch. The other compartment 19receives the Various components of the switch and its magnetic circuitswhich are mounted generally adjacent the sensing face 20 of the case.

The switch components include a switch element 22 of the encapsulatedreed type having a pair of magnetically operable contact elements 24 and25 mounted in a sensing magnetic field and in biasing magnetic field.The sensing magnetic field is formed by a pair of sensing magnets 26 and28 which are mounted in the case in spaced relation to each other withthe north pole of magnet 26 adjacent the sensing face 20 and the southpole of magnet 28 adjacent the sensing face. These sensing magnets arepreferably of barium ferrite, a material of low permeability so that theflux from these sensing magnets is greatly increased by the presence ofa ferromagnetic object 30 adjacent the sensing face 20. The switchelement 22 is mounted adjacent the sensing magnets 26 and 28 with themagnetically operable contacts 24 and 25 of the switch element in thefield between the other pair of poles of the sensing magnets.

Biasing magnets 32 and 34 are mounted along the side of the switchelement 22 opposite from the sensing magnets, and a spacer strip 36 ofnonmagnetic material is preferably installed intermediate the biasingmagnets and switch element. Relative polarity of the biasing to thesensing magnets is opposite as indicated-4n other words a north pole ofa biasing magnet opposes a south pole of a sensing magnet and viceversa.

A shield plate 40 of ferromagnetic material is mounted in the case withthe biasing magnets 32 and 34 resting on one face 41 thereof, and isheld in position by a spacer 42, which may be a length of plastic tubinginserted between the other face of the plate and the bafiie 11 of thecase. The shield plate 40 forms a closed flux path, extending parallelto the sensing face, for the fields of both the biasing and sensingmagnets and hence prevents or limits the distance to which the biasingand sensing magnetic fields can extend inwardly from the sensing face.Any extraneous magnetic material which does not extend toward thesensing face beyond a plane slightly spaced from the inner face 44 ofthe plate 40 will not affect the sensing and biasing magnetic fields.This is illustrated in FIG. 2 wherein the switch is shown attached bymounting bolts 15 directly to a ferromagnetic structural member 46, theend 48 of which does not extend beyond the plane referred to above.Therefore, this structural member, or any other object which does notproject beyond this plane will not affect the desired operatingcharacteristics of the switch, which may be set when the switch ismanufactured as illustrated by the construction of FIGS. 1 and 2, orwhich may be varied in use as illustrated by the construction in FIGS. 3and 4.

In the construction of FIGS. 1 and 2, the switch is assembled byinserting the sensing magnets 26 and 28 into pockets provided inthecase, and the switch element 22, spacer 36, biasing magnets 32 and 34,shield plate 40, and spacer tube 42 are all slipped into position. Withthe various parts so assembled, the biasing magnets 32 and 34 aremounted for movement toward and away from each other on the surface 41of the plate 40. As the biasing magnets 32 and 34 are moved toward eachother, the biasing magnetic field becomes increasingly predominant inits effect upon the switch contact elements 24 and 25, and as thebiasing magnets are moved away from each other the field of the sensingmagnets becomes predominant.

For example, if it is desired to have the switch contacts 24 andnormally open and to close in the presence of a ferromagnetic object ata distance of one-half inch from the sensing face 20 of the case, thissetting is obtained as follows: The object 30 is placed one-half inchaway from the sensing face and the biasing magnets are moved together sothat the distance separating them is zero. A continuity check shouldshow that the switch contacts are closed. The biasing magnets are thengradually moved apart until the switch contacts open and then close. Theobject 30 is removed, brought back to the desired one-half inch spacingand the switch contacts should close. The switch is then potted byfilling the compartment 16 of the case with a suitable plastic material.

This adjustment of the normal position of the switch contacts and thesensitivity thereof to a change from normal posit-ion as the result ofthe presence of a magnetic object adjacent the sensing face of the caseis obtained by varying the length of the flux path of the biasingmagnetic field through the biasing magnets 32 and 34, the shield memberand the magnetically operable switch contacts 24 and 25. In theconstruction shown, the length of the flux path is varied by means ofadjusting the relative position of the biasing magnets along the face 41of the shield plate, and while this is preferable because of thesimplicity of construction, the flux path length could obviously also bechanged by varying the position of the shield plate.

The proximity switch construction shown in FIGS. 3 and 4 is similarexcept that means are provided for adjusting the position of the biasingmagnets 32 and 34 at any time during the life of the switch, no pottingcompound being placed in the switch compartment 16 of the case and theadjusting means being accessible externally of the case. A spring isplaced between the biasing magnets and normally urges them apart, whilepositioning screws 52 and 53, each extending through a threaded boss 54in the case, enable the biasing magnets to be moved in opposition to theforce of the spring 50. Since no potting compound is used, all thecomponents of the switch except the biasing magnets may be fixed inposition in the case by cementing or other suitable means. A tongue andgroove connection 56, 57 is employed between the biasing magnets 32 and34 and the shield plate 40, the biasing magnets being urged against theface of the shield plate by a combination spacer and retainer member 58.

While preferred embodiments have been described above in detail, it willbe understood that numerous modifications might be resorted to withoutdeparting from the scope of the invention as defined in the followingclaims.

I claim:

1. A magnetic proximity switch including a nonmagnetic case having asensing face and a switch element with a pair of magnetically operablecontacts mounted in a sensing magnetic field and in a biasing magneticfield; characterized by the sensing magnetic field being formed by apair of sensing magnets mounted in the case in spaced relation to eachother with one pair of opposite poles adjacent the sensing face, themagnetically operable contacts of the switch element being mounted inthe field between the other pair of opposite poles; the biasing magneticfield being formed by a pair of biasing magnets positioned in the casewith one pair of opposite poles adjacent the magnetic ally operablecontacts of the switch element; a shield member of magnetic materialmounted in the case to form a closed flux path for the sensing andbiasing magnetic fields, which closed flux path is spaced from thesensing face and shields both the sensing and biasing magnetic fieldsfrom the influence of extraneous magnetic material other than magneticmaterial to be detected adjacent the sensing face of the case.

2. A magnetic proximity switch as claimed in claim 1 furthercharacterized by means for varying the length of the flux path of thebiasing magnetic field through the biasing magnets, the shield member,and the magnetic operable switch contacts.

3. A magnetic proximity switch as claimed in claim 2 wherein the shieldmember and biasing magnets are mounted for movement relative to eachother and to the magnetically operable switch contacts.

4. A magnetic proximity switch as claimed in claim 1 wherein the shieldmember is provided with a surface on which the biasing magnets aremounted for movement toward and away from each other to vary the biasingmagnetic field relative to the magnetically operable switch contactswhereby the normal position of the switch contacts can be varied and thesensitivity of the proximity switch to magnetic material adjacent thesensing face of the case can be adjusted.

5. A magnetic proximity switch according to claim 4 furthercharacterized by means accessible externally of the case for adjustingthe position of the biasing magnets relative to each other.

6. A magnetic proximity switch as claimed in claim 5 wherein the meansfor adjusting the relative position of the biasing magnets includes aspring normally urging the biasing magnets apart, and positioning meansextending through the case for moving the biasing magnets together inopposition to the force of the spring.

7. A magnetic proximity switch according to claim 1 wherein the sensingmagnets are each of low permeability.

8. A magnetic proximity switch according to claim 1 wherein the shieldmember is a plate extending parallel to the sensing face of the casewit-h the sensing magnets, switch element and biasing magnet beingmounted between one face of the plate and the sensing face of the case.

9. A magnetic proximity switch according to claim 8 wherein the biasingmagnets of each mounted on the said one face of the plate forming theshield member for movement toward and away from each other.

10. A magnetic proximity switch as claimed in claim 9 furthercharacterized by adjusting means extending through the case for varyingthe position of at least one of the biasing magnets relative to themagnetically operable contacts of the switch element.

' References Cited UNITED ROBERT K. SCHAEFER, Primary Examiner.

M. GINSBURG, Assistant Examiner.

1. A MAGNETIC PROXIMITY SWITCH INCLUDING A NON-MAGNETIC CASE HAVING ASENSING FACE AND A SWITCH ELEMENT WITH A PAIR OF MAGNETICALLY OPEARBLECONTACTS MOUNTED IN A SENSING MAGNETIC FIELD AND IN A BIASING MAGNETICFIELD; CHARACTERIZED BY THE SENSING MAGNETIC FIELD BEING FORMED BY APAIR OF SENSING MAGNETS MOUNTED IN THE CASE IN SPACED RELATION TO EACHOTHER WITH ONE PAIR OF OPPOSITE POLES ADJACENT THE SENSING FACE, THEMAGNETICALLY OPERABLE CONTACTS OF THE SWITCH ELEMENT BEING MOUNTED INTHE FIELD BETWEEN THE OTHER PAIR OF OPPOSITE POLES; THE BIASING MAGNETICFIELD BEING FORMED BY A PAIR OF BIASING MAGNETS POSITIONED IN THE CASEWITH ONE PAIR OF OPPOSITE POLES ADJACENT THE MAGNETICALLY OPERABLECONTACTS OF THE SWITCH ELEMENT; A SHIELD MEMBER OF MAGNETIC MATERIALMOUNTED IN THE CASE TO FORM A CLOSED FLUX PATH FOR THE SENSING ANDBIASING MAGNETIC FIELDS, WHICH CLOSED FLUX PATH IS SPACED FROM THESENSING FACE AND SHIELDS BOTH THE SENSING AND BIASING MAGNETIC FIELDSFROM THE INFLUENCE OF EXTRANEOUS MAGNETIC MATERIAL OTHER THAN MAGNETICMATERIAL TO BE DETECTED ADJACENT THE SENSING FACE OF THE CASE.